1. Field of the Invention
The present invention relates to FSK (frequency shift keying). More particularly, it relates to a noise-removing technology upon FSK demodulation. One example of the FSK demodulation is Caller ID service which is one of telephone services. In this service, FSK is used for performing data communication of the caller ID.
2. Description of the Prior Art
As one of telephone services, there is a Caller ID service. In this service, data communication of the caller ID is performed according to the BELL 202 standard and the CCITT V.23 standard. In these standards, for the noise present on a telephone line, the ratio of a FSK signal to a noise is defined as 20 [dB]. Whereas, the amplitude of the FSK signal to be received is defined as xe2x88x9232 [dBm] to xe2x88x9212 [dBm]. For this reason, in the data communication of the caller ID, the noise amplitude to be removed varies according to the FSK signal amplitude.
As a demodulation method of FSK, there is a zero-crossing method. With the zero-crossing method, the interval between the crossings across the line at an amplitude of zero of the FSK signal, i.e., the time of half period is determined to discriminate between two carrier frequencies. With such a method for determining the half period, the amplitude level of the FSK signal is not involved in demodulation. Therefore, the noise in the same frequency band as the FSK signal is demodulated.
For preventing the noise from being demodulated as a signal, in the prior-art method, the noise amplitude level to be removed is preset, and an amplitude smaller than the amplitude level is regarded as a noise, and removed. However, even in this case, when the noise amplitude also varies in accordance with the input signal amplitude as the Caller ID service, it is not possible to completely remove the noise, or even the FSK signal which should not be removed is erroneously removed.
When the Caller ID is transmitted, a noise is also present together with the FSK signal. However, with a prior-art method, it is not possible to discriminate between the FSK signal and the noise. As a result, the noise has been erroneously demodulated. Further, with the prior-art method, it is not possible to detect the completion of FSK. With the prior-art method, whether it is the Caller ID or a noise has been judged based on the demodulation result.
With the prior-art method, for removing a noise from the FSK signal, the component having a smaller amplitude than the preset amplitude is removed as the noise. Such a prior-art method may not be applicable to removal of the noise present during a period in which the FSK signal is not transmitted, and varying in amplitude in accordance with the FSK signal amplitude. This is for the following reason. The minimum amplitude of the signal may be smaller than the amplitude of the noise at the time of the maximum signal amplitude. In this case, not only the noise but also even the signal are removed.
Further, with digital FSK, respective digital values of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d are set to correspond to either of two different frequencies, however, the two signals may be different in amplitude from each other. In such a case, the FSK signal having a smaller amplitude in accordance with the FSK signal amplitude may be erroneously removed as a noise.
Object of the Invention
An object of the present invention is to provide a FSK demodulation system capable of changing the noise amplitude to be removed in accordance with the amplitude level of the input signal.
A FSK demodulation system of the present invention has a means for comparing two preset values across 0, a positive side level shift amount and a negative side level shift amount, with an inputted amplitude level; and a demodulation means for performing demodulation based on the comparison result.